1. Field of the Invention
This invention relates to equipment for manufacture of semiconductor devices using photolithographic masks/reticles for patterning wafers/substrates and to measurement of levels of the reticle relative to the wafer, and more particularly to leveling a mask used in a stepper.
2. Description of Related Art
In the manufacture of VLSI chips, the long term trend is that dimensions of the devices in the contemporary manufacturing environment are continuously shrinking to smaller and smaller dimensions. A problem arises as dimensions shrink more and more because the surface of a work-piece such as a semiconductor wafer with layers formed on the surface thereof is not flat. Thus, even assuming that a reticle held in a wafer stepper is perfectly flat, because of the lack of flatness of the surface of the workpiece surface there will be a variation in spacings between the reticle and the surfaces of the photolithographic material or other materials which lie immediately below the various parts of the "flat" reticle. These variations become increasingly more important at the dimensions shrink. For example, during exposure of the newly deposited photolithographic masking material formed on the workpiece, it is necessary in order to achieve anything close to a correct focus of the radiant energy which passes through the reticle to the workpiece, that the variations in levels of the workpiece surface relative to the reticle be minimized in the areas being exposed at a given time. Accordingly it is necessary to have a technique which affords the manufacturer an ability to determine the degree to which the reticle has been leveled relative to the surface of the workpiece below it so that adjustments can be made to correct the performance of the stepper.
Steppers are step-and-repeat lithographic system cameras in which a workpiece is exposed over and over by two dimensional translations of the workpiece beneath the reticle and the lens (if any) of the stepper which use a reticle as the object whose image is projected either directly in a 1:1 projection system or through a stepper lens upon the surface of a workpiece with the reduced image projected by the stepper. In the case of the reduced image system pattern transfer from the reticle to the mask on the wafer is accomplished by optically reducing the image to the correct dimensions while simultaneously focusing it onto a specified region of the wafer. After each exposure, the process continues with the wafer being translated to the next desired imaging position where the image is exposed again in the same way. The image is exposed, in that way, over and over again until the desired regions of the work-piece have been exposed with the image on the reticle. In each position of the workpiece, the process of leveling must be repeated to achieve the best possible focus of the reticle image upon the region of the workpiece being exposed.
Use has been made in the past of a line/space pattern to check stepper leveling performance. A problem with that approach is that a great deal of time is required to measure Critical Dimension (CD) value by means of Scanning Electron Microscopy (SEM.)